Numerous prior art devices require the use of an analog switch that can pass or block a high voltage input signal. An example of such a device is depicted in FIG. 1. Medical ultrasound system 100 comprises a plurality of probes, here labeled as exemplary probes 131, 132, 133, and 134. Additional probes may be utilized. Each probe is coupled to one or more sets of an analog switch and transducer. In this example, probe 131 is coupled to analog switches 111, 112, 113, and 114, each of which receives high voltage input signal 140 from probe 131. Analog switch 111 is coupled to transducer 121, analog switch 112 is coupled to transducer 122, analog switch 113 is coupled to transducer 123, and analog switch 114 is coupled to transducer 124 Additional sets of an analog switch and transducer can be utilized. The role of each of the analog switches 111, 112, 113, and 114 is to connect or disconnect the probe (such as probe 131) to the switch's respective transducer.
An example of a prior art analog switch is shown in FIG. 2. Analog switch 111 receives high voltage input signal 140, which in this example ranges from +100V to −100V. Analog switch 111 comprises transistors 211, 212, 240, 261, and 262 as depicted. Transistors 261 and 262 receive control signal 270. Control signal 270 turns analog switch 111 on or off. When control signal 270 is low, transistor 261 (which is a PMOS transistor) turns on and its drain is pulled to VPP (which here is +100V), and transistor 262 (which is an NMOS transistor) turns off. When control signal 270 is high, transistor 261 turns off, and transistor 262 turns on and its drain is pulled down to VNN (which here is −100V).
The drain of transistor 261 and the drain of transistor 262 form a node that connects to the gates of transistors 211 and 212. When control signal 270 is low, transistors 211 and 212 will turn on, which will allow high voltage input signal 140 to be sent to transducer 121.
When control signal 270 is high, transistors 211 and 212 will turn off. Control signal 270 will turn on transistor 240, which will pull the node between transistors 211 and 212 down to VNN (which is −100V). Transistor 240 provides a shunt termination function and improves the off-isolation of switch 111. Notably, however, transistor 240 requires a high-voltage negative supply (here, VNN=−100V).
There are numerous drawbacks with the prior art design of FIG. 2. First, it requires significant board space and has a complicated board design and layout, which results in high manufacturing costs. Second, the presence of high DC voltages (+/−100V) in the probe could cause a bad shock for the operator or patient. Third, the system requires a low-impedance, high-voltage power cord that is neither thin nor flexible. Fourth, a stringent power-up sequence must be followed to prevent transistor breakdown and latch-up. Fifth, the system requires the use of thick gate oxide and may not be feasible with nominal BCD (Bipolar-CMOS-DMOS) processes.
The prior art includes one attempted solution. FIG. 3 depicts analog switch 300. Under this design, the positive high voltage supply can be lowered down to 5V instead of 100V if transistors 310 and 320 are low threshold transistors and their gates are clamped to their sources with zener diode 330. However, analog switch 300 still requires a high negative voltage (here, −100V) for the shunt termination function, which still causes the drawbacks described previously.
What is needed is an improved analog switch design that can transmit high voltages to a load without requiring high voltage DC power supplies.